Fish with cleft lip solves evolution riddle

A 395-million-year-old fish may have answered a pressing question of human evolution: how did our nasal cavities adopt their current layout? The strange specimen has nostrils in the middle of its upper teeth.

The fish, called Kenichthys campbelli, represents a halfway point in the evolutionary reshuffling of the nasal passages, say Min Zhu of the Chinese Academy of Sciences in Beijing and Per Ahlberg of Uppsala University in Sweden, who describe the fossil in this week's Nature1. Most modern fish have four external nostrils, whereas land vertebrates, which are descended from Kenichthys, have nasal passages that form openings near the throat called choanae.

Some experts suggested that choanae evolved through the gradual repositioning of external nostrils. But sceptics countered that this would involve the nostrils migrating through the line of the teeth, a feature not seen in any fossil.

Until now, that is. Kenichthys's cleft lip catches evolution in the act of moving the nostrils back through the teeth and palate, say Zhu and Ahlberg, effectively ending a debate that has persisted for around a century. The fossil, found in Yunnan in China, represents a crucial intermediate step between external nostrils and choanae.

This theory is also bolstered by the fact that no known animal has both choanae and a double pair of external nostrils. So it looks like Kenichthys was the unlucky link in the chain that had its teeth rearranged by a migrating nostril. "Any other hypothesis would require some ad hoc explanation of why this morphology looks intermediate," comments Zhu.